Robert Hatch

SUBTOPIC:

 Demonstration of Newton’s Three Laws of Motion and the Law of Gravitation

OBJECTIVES:

The students will:

1. give examples of each of Newton’s three laws as they occur in everyday experiences
2. visualize and differentiate the difference between a direct proportion and an inverse proportion using the formula F=ma
4. understand how the gravitational law affects the tides of the earth
5. calculate the actual gravitational force between the sun and earth and the moon and earth to conclude which has the stronger influence

Background Information

Isaac Newton summed up motion in three laws. Today we take these laws for granted as we grow up assuming they are true. We do not realize the struggles scientists went through in attempt to understand the world around them. The following activities use brainstorming, discussion, and simple labs to illustrate the laws.

Newton’s Three Laws:

1. An object which is moving at a constant velocity or at a state of rest does not change its state unless a force acts upon it.
2. Acceleration of an object increases as the amount of force causing the acceleration increases when mass is constant.
3. For every force, there is an equal and opposite force.

ACTIVITY #1:

Newton’s First Law

            20 min

MATERIALS:

 CHALK AND BLACKBOARD

PROCEDURE:

1. Brainstorm everyday examples of the first law.
2. Present a lecture to students, including the following necessary background information:

   Just prior to Newton’s time Galileo had worked with the idea of acceleration. Galileo could only guess about time since precise clocks had not been invented. This is why he rolled metal balls down smooth ramps. Since he noticed how a ball slowed when rolling across the floor, he concluded that friction was the cause. Thus friction was responsible for the idea that objects in motion naturally come to rest. But ‘rest’ is just one kind of constant velocity. The concept of inertia and Newton’s 1st law emerged from this insight.

3. Use some of the following examples to explain to the students how Newton’s first law occurs in everyday events:
   1. car suddenly stops and you strain against the seat belt
   2. when riding a horse, the horse suddenly stops and you fly over its head
   3. the magician pulls the tablecloth out from under a table full of dishes
   4. the difficulty of pushing a dead car
   5. lawn bowling on a cut and rolled lawn verses an uncut lawn
   6. car turns left and you appear to slide to the right

ACTIVITY #2 :

Relationships in Newton’s Second Law

           20 minutes

MATERIALS:

 METER STICKS, INDEX CARDS, TAPE, MARKERS

PROCEDURE:

1. Newton’s second law deals with F=MA. When written A = F/M on sees that the acceleration will vary directly with the force applied and inversely with the mass of the body. Since students have trouble with these terms, a simple visual aid can help them.
2. Take three index cards and write an A, F, and M on them, and then tape the F card to a meter stick at the 50 cm mark so that it hangs down. Next tape the A card at 0 cm and the M card at the 100 cm mark.
3. Explain to the students that if the force is constant (either flip the card up or cover it with your hand), when acceleration increases (raise the 0 cm end of the meter stick at a 30 angle) mass must decrease.
4. Note that the 100 cm end now angles down. This shows an inverse proportional relationship.
5. Now cover the acceleration card with your hand. When force or mass increases or decreases the other variable will do the same. This shows the direct proportional relationship.
6. Lastly, do the same for the M card.
7. Plug in numbers and work through some simple F=MA problems.
8. Use the meter stick to help visualize what the answer will be (greater or smaller). Finally brainstorm everyday applications, some examples are listed below.
   1. hitting a baseball, the harder the hit, the faster the ball goes
   2. accelerating or decelerating a car
   3. The positioning of football players – massive players on the line with lighter (faster to accelerate) players in the backfield
   4. a loaded versus an unloaded truck

ACTIVITY #3:

Constant Force Increases Speed

            25 minutes

MATERIALS:

 SKATEBOARD AND SPRING SCALE

PROCEDURE:

1. Have a student bring in a skateboard.
2. Have one student stand on the skateboard at the front of the class and hold one end of the spring scale.
3. Another student should pull the first student at a constant force of 10 newtons.
4. Observe the speed of the students as they keep the force constant.
5. Explain that this shows the direct relationship between force and acceleration.

ACTIVITY #4:

Newton’s Third Law

            10 minutes

MATERIALS:

 CHALK AND BLACKBOARD

PROCEDURE:

1. Brainstorm everyday examples of the third law with the class. Listed below are some examples:
   1. rockets leaving earth–many physicists of the nineteen hundreds (Goddard’s time) said that rockets could never leave the earth. Discuss how a spaceship flies in space.
   2. guns being fired- discuss why they kick in proportion to the size of the bullet. Why is the stock of the rifle so large? What would happen if the stock of a shotgun came back to a point shape?
   3. two cars hit head on
   4. astronauts in space
   5. pool or billiards
   6. jumping out of a boat onto the dock
   7. sprinklers rotating

ACTIVITY #5:

Balloon Races

           50 minutes

MATERIALS:

 3 LONG BALLOONS, 1 PLASTIC STRAW, 60 CM (OR MORE) OF FINE WIRE OR FISHING LINE, TAPE, MODEL AIRPLANE (BALSA WOOD), MARKER

PROCEDURE:

1. Have the students follow the procedures listed below:
   1. Blow up balloons, fasten them with rubber bands, and label them A, B, and C.
   2. Tape the straw lengthwise to Balloon B and run the wire through the straw.
   3. Tape Balloon C to the top of the fuselage of the model airplane, placing the balloon opening toward the tail of the airplane.
   4. Loosen the rubber band on Balloon A very slowly and record the speed and direction of movement.
   5. Now, on Balloon B, have a partner hold each end of the wire through the straw and keep the wire tight.

      Cut the rubber band quickly and observe the balloon. Record observations. 

   6. Draw a sketch showing the direction the air in the balloon moved. Also, on the same sketch, draw a diagram of how the balloon moved along the wire.
   7. To test Balloon C, have a partner hold the airplane loosely. Cut the rubber band as your partner releases the airplane. Record the flight. You may wish to stage olympic type competitions between lab partners.

   Hint: Try different shapes of balloons. Elongated should work the best. Also a round trip rocket could be designed.

2. Have the students answer the following questions:
   1. Describe the reaction of the rubber band when it was cut.
   2. Describe the flight of Balloon B.
   3. What was the force that moved the Balloon B?
   4. Why did Balloon B move differently from Balloon A?
   5. Why did Balloon B and Balloon C move more rapidly than Balloon A?
   6. State Newton’s third law and explain how this activity illustrates it.

ACTIVITY #6:

A Reverse World

            20 – 30 minutes

MATERIALS:

 PENCIL AND PAPER

PROCEDURE:

1. Ask the students to write a 2 to 3 page science fiction story describing what differences we would observe if the opposite of Newton’s three laws were true on earth. For example, guns would not have recoil, and a cannon’s mass would not have to be greater than a cannon ball. You would also not be pushed back in your seat when undergoing acceleration in a car.
2. As an alternative, you may wish to do a verbal brainstorming of how things on earth would be different if we lived under the reverse of Newton’s laws.

ACTIVITY #7:

What Really Causes the Tides

            35 minutes

MATERIALS:

 CHALK AND BLACKBOARD

Background Information for Activity

The gravitational force of the moon and sun play an important role in the tides. When the sun, earth, and moon are in a straight line, their combined gravitational pull causes extra high and low tides known as spring tides. Whenever there is a full or new moon this occurs. The neap tides form when the sun, the earth, and the moon form a right angle, causing a half moon. The question is which, the sun or the moon, has the stronger gravitational pull?

PROCEDURE:

1. Using Newton’s gravitational formula, have the students research (homework) the data needed and do a class project at the board doing the calculations.
2. Depending on the ability of the students, each student may do their own calculations.
   Mass of Earth 5.98 x 10²⁴ Kg

   Mass of Sun 1.98 x 10³⁰ KgMass of Moon 7.36 x 10²² Kg

   Distance – Earth to Sun 1.50 x 10¹¹ m

   Distance – Earth to Moon 3.84 x 10⁸ m

   F = G m₁m₂ where G = 6.67 x 10^-11 m³

   d² Kg sec₂

   1. Sun to Earth

      F = 6.67 x 10^-11 m³ x 1.98 x 10³⁰ Kg x5.98 x 10²⁴ Kg

      Kg sec² (1.55 x 10¹¹ m)²
      F = 3.51 x 10²² m/sec²

   2. Moon to Earth

      F = 6.67 x 10^-11 m³ x 7.36 x 10²² Kg x 5.98 x 10²⁴ Kg

      Kg sec² (3.84 x 10⁸ m)²

      F = 1.99 x 10¹⁹ m/sec²

3. Explain to the students that the sun, therefore, should have greater pulling power. The tidal bulge produced by the sun is 46% of that produced by the moon. The tides are primarily caused by the gravitational pull of the moon. Besides the ocean tides, the moon also causes tides in the solid body of the earth as much as 25 cm. These earth tides are very hard to observe or detect. The water on the side of the earth near the moon is pulled toward the moon with a greater than average force, the water on the far side is pulled with a less than average force. In addition, the rotation of the earth helps raise a tidal bulge on the side away from the moon. Thus, two bulges appear in the water on opposite sides of the earth. Tidal bulges occur 3 ahead of the line which runs between the centers of the earth and the moon.

   The pull between the sun and the earth is about 180 times stronger than the pull between the moon and the earth. So our calculations are correct, but why doesn’t the sun cause tides 180 times greater? Because of the sun’s great distance from the earth, there is not much difference in the distances from the sun to the earth’s near and far side. This means that there is not much difference in the gravitational pull of the sun on the ocean nearest it and on the ocean furthest from it. The relatively small difference in pulls on the opposite sides of the earth only slightly elongates the earth’s shape. Thus the sun produces tidal bulges less than those of the moon.

   The tilt of the earth also affects tides. The tilt causes the 2 daily high tides experienced in most parts of the ocean to be unequal in height.

ACTIVITY #8:

Additional Gravity Calculation for Honors Students

           5 minutes

PROCEDURE:

1. Since Jupiter is 7.8 x 10¹¹m from the sun and has a mass of 1.8 x 10²⁷Kg. Have the students calculate Jupiter’s gravitational force, and determine if the sun produces tides on Jupiter.
   F = 6.67 x 10^-11 m³x 1.98 x 10³⁰ Kg x 1.8 x 10²⁷ Kg

   Kg sec² (7.8 x 10¹¹ m)²

   F = 3.9 x 10²³ m/sec²